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Synthetic Studies of Nitrogen-Containing Heterocycles Under Microwave Irradiation, Bruno Nascimento, Coimbra, 2013

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Synthetic Studies of Nitrogen-Containing Heterocycles Under Microwave Irradiation, Bruno Nascimento, Coimbra, 2013 Imagem Bruno Filipe Oliveira Nascimento Synthetic Studies of Nitrogen-Containing Heterocycles under Microwave Irradiation Tese orientada pelo Professor António Manuel d'Albuquerque Rocha Gonsalves e pela Professora Marta Piñeiro Gómez e apresentada na Universidade de Coimbra para obtenção do grau de Doutor em Química com especialidade de Síntese Orgânica July 2013 Bruno Filipe Oliveira Nascimento Synthetic Studies of Nitrogen-Containing Heterocycles under Microwave Irradiation Tese orientada pelo Professor António Manuel d'Albuquerque Rocha Gonsalves e pela Professora Marta Piñeiro Gómez e apresentada na Universidade de Coimbra para obtenção do grau de Doutor em Química com especialidade de Síntese Orgânica July 2013 Aos elementos da FREQ, Frente Revolucionária do Enclave das Químicas. Obrigado pela longa e intensa amizade... Aquele abraço! Contents Preface xiii Abstract xiv Resumo xvi Listing of Abbreviations xviii Listing of Symbols xxi Listing of Schemes xxii Listing of Figures xxvi Listing of Tables xxviii Nomenclature xxix 1. Microwave Chemistry 1 I. Introduction & Relevance 1 II. Microwave Fundamentals 2 A. Microwave Radiation 2 B. Dielectric Heating 3 C. Dielectric Properties 5 D. Microwave versus Conventional Heating 7 E. Microwave Effects 8 1. Thermal/Kinetic Effects 8 2. Specific Microwave Effects 9 3. Non-Thermal Microwave Effects 12 III. Microwave Equipment 13 A. Domestic Microwave Ovens 14 B. Dedicated Microwave Reactors 15 C. CEM Discover S-Class 16 IV. References 17 2. Pyrroles 23 I. Introduction & Relevance 23 II. Classical Synthetic Methods 25 A. Paal-Knorr Synthesis 25 B. Knorr Synthesis 25 C. Hantzsch Synthesis 26 III. Microwave-Assisted Synthetic Methods 26 A. Literature Review & Selected Examples 27 B. Paal-Knorr Synthesis of 2,5-Dimethyl-1H-Pyrroles 31 |ix Contents C. Paal-Knorr Synthesis of Bis-2,5-Dimethyl-1H-Pyrroles 33 D. Multicomponent Synthesis of 3,5-Diaryl-2-Methyl-1H-Pyrroles 34 IV. Summary 41 V. References 42 3. Porphyrins & Hydroporphyrins 45 I. Introduction & Relevance 45 II. Classical Synthetic Methods 47 A. Porphyrins 47 1. Rothemund Synthesis 48 2. Adler-Longo Synthesis 48 3. Rocha Gonsalves Two-Step Synthesis 49 4. Lindsey Two-Step Synthesis 49 5. Rocha Gonsalves One-Step Synthesis 49 6. Other Syntheses 50 B. Hydroporphyrins 51 1. Reduction of Porphyrins 52 2. Oxidation of Porphyrins 52 3. Cycloaddition of Porphyrins 53 4. Oxidation of Porphyrinogens 54 5. Other Syntheses 54 III. Microwave-Assisted Synthetic Methods 55 A. Literature Review & Selected Examples 55 1. Porphyrins 55 2. Hydroporphyrins 58 B. Synthesis of meso-Tetraarylporphyrins 59 C. Synthesis of meso-Tetraarylhydroporphyrins 64 IV. Summary 67 V. References 68 4. Hantzsch 1,4-Dihydropyridines 71 I. Introduction & Relevance 71 II. Classical Synthetic Methods 72 III. Microwave-Assisted Synthetic Methods 75 A. Literature Review & Selected Examples 75 B. Multicomponent Synthesis of Hantzsch 1,4-Dihydropyridines 83 C. Oxidation of Hantzsch 1,4-Dihydropyridines 85 x| Contents IV. Summary 88 V. References 88 5. Biginelli 3,4-Dihydropyrimidines 93 I. Introduction & Relevance 93 II. Classical Synthetic Methods 96 III. Microwave-Assisted Synthetic Methods 99 A. Literature Review & Selected Examples 99 B. Multicomponent Synthesis of Biginelli 3,4-Dihydropyrimidines 106 C. Multicomponent Synthesis of Biginelli Bis-3,4-Dihydropyrimidines 111 D. Synthesis of Biginelli-Type 3,4-Dihydropyrimidine-2(1H)-Thiones 112 E. Oxidation of Biginelli 3,4-Dihydropyrimidines 118 IV. Summary 126 V. References 127 6. Experimental 131 I. Instrumentation 131 A. Microwaves 131 B. Melting Points 131 C. Elemental Analysis 131 D. Ultraviolet-Visible Absorption Spectroscopy 131 E. Nuclear Magnetic Resonance Spectroscopy 131 F. Gas Chromatography-Mass Spectrometry 131 G. Mass Spectrometry 131 H. X-Ray Diffraction 131 II. Materials 132 A. Reagents 132 B. Solvents 132 C. Others 132 III. Methods 132 A. Pyrroles 132 1. Paal-Knorr Synthesis of 2,5-Dimethyl-1H-Pyrroles 132 2. Paal-Knorr Synthesis of Bis-2,5-Dimethyl-1H-Pyrroles 133 3. Multicomponent Synthesis of 3,5-Diaryl-2-Methyl-1H-Pyrroles 134 4. Claisen-Schmidt Synthesis of Chalcones 139 5. Vilsmeier-Haack Acetylation of Pyrrole 142 B. Porphyrins 142 |xi Contents 1. Synthesis of meso-Tetraarylporphyrins 142 i. One-Step Methodology 142 ii. Two-Step Methodology 147 C. Hydroporphyrins 147 1. Synthesis of meso-Tetraarylbacteriochlorins 147 2. Synthesis of meso-Tetraarylchlorins 148 D. Hantzsch 1,4-Dihydropyridines 149 1. Multicomponent Synthesis of Hantzsch 1,4-Dihydropyridines 149 2. Oxidation of Hantzsch 1,4-Dihydropyridines 153 i. Heterogeneous Oxidative Aromatisation 153 ii. Homogeneous Oxidative Aromatisation 153 E. Biginelli 3,4-Dihydropyrimidines 156 1. Multicomponent Synthesis of Biginelli 3,4-Dihydropyrimidines 156 2. Multicomponent Synthesis of Biginelli Bis-3,4-Dihydropyrimidines 164 3. Synthesis of Biginelli-Type 3,4-Dihydropyrimidine-2(1H)-Thiones 166 4. Oxidation of Biginelli 3,4-Dihydropyrimidin-2(1H)-Ones 168 F. Spectral & Photophysical Studies 170 G. Cytotoxicity Studies 171 IV. References 171 xii| Preface “By three methods we may learn wisdom: first, by reflection, which is noblest; second, by imitation, which is easiest; and third, by experience, which is bitterest.” Confucius (551 - 479 BC) The work presented in this dissertation was carried-out at the Research Laboratory on Organic Chemistry of the Department of Chemistry, Faculty of Sciences and Technology of the University of Coimbra, Portugal, between January 2008 and June 2012, and was by no means accomplished in an individual manner, but through several and fruitful interactions. Hence, it is of the essence to acknowledge the valuable contributions of all persons and entities involved. To Prof. Marta Piñeiro Gómez, my supervisor, I acknowledge the enlightened and informal scientific guidance, always characterised by a generous amount of patience and good-humour, which was utterly determinant throughout this project. To Prof. António M. d'A. Rocha Gonsalves, my co-supervisor, I acknowledge the thoughts and opinions, always furnished in a singular and charismatic fashion, that were essential to the successful scrutiny of several queries. To Prof. Teresa M. V. D. Pinho e Melo, head of the Research Laboratory on Organic Chemistry, I acknowledge the useful clarifications that were fundamental to the investigation of various questions. I am profoundly grateful to the following persons for their expertise and availability, regarding the technical features of the structural characterisation, spectral, photophysical and cytotoxicity studies of some of the compounds synthesised in this work: Prof. Maria Elisa S. Serra (Elemental Analysis), Pedro Cruz and Prof. Rui M. M. Brito (Nuclear Magnetic Resonance Spectroscopy), Júlio Sampaio (High-Resolution Mass Spectrometry), Alexandra Gonsalves (Mass Spectrometry), Sílvia Gramacho (Gas Chromatography-Mass Spectrometry), Prof. José A. Paixão (X-Ray Diffraction), Daniela Pinheiro, João Pina and Prof. J. Sérgio Seixas de Melo (Spectral and Photophysical Studies), Mafalda Laranjo, Ana Abrantes and Prof. Maria Filomena Botelho (Cytotoxicity Studies). I wish to convey my deepest gratitude to all my laboratory co-workers, for their support, team-spirit and helpful sharing of ideas. In particular, I would like to thank Prof. Arménio C. Serra for the constant, proficient and good-humoured exchange of opinions, in spite of our differences at the musical level and, consequently, our customary disagreement concerning the frequency setting of the laboratory radio. I would also like to express recognition to my colleagues Cláudio Nunes, Nelson Pereira, Rui Nunes and Salomé Santos for their friendship, encouragement and the always riveting discussions, scientific, political or other, particularly when the best results of this work were not being achieved at the desired rate. Furthermore, thanks are due to Rita Navarro, for reading and correcting part of this manuscript and providing me with both precious and pertinent suggestions. Lastly, I wish to deeply acknowledge my parents, Fátima and Pedro, for their continual affection, endless support and, as long as I can remember, for instigating my free-will and freedom of thought. Financial aid provided by Chymiotechnon, Coimbra Chemistry Centre, University of Coimbra and, particularly, Fundação para a Ciência e Tecnologia, which kindly presented me with a Ph.D. grant (SFRH/BD/QUI/41472/2007), is also gratefully appreciated. |xiii Abstract The central goal of the work presented in this doctoral dissertation was the application of microwave irradiation to the development of efficient, straightforward and reproducible synthetic methods of various interesting and broadly recognised nitrogen-containing heterocycles. Their reactivity under microwave heating conditions, particularly in oxidation processes, was also studied, inexpensive, undemanding and environment- friendly synthetic strategies being employed whenever possible. The illustrious Paal-Knorr synthesis of pyrroles was revised, some 2,5-dimethyl-1H-pyrroles and bis-2,5- dimethyl-1H-pyrroles being readily prepared with high reaction yields through a solventless and microwave- activated procedure. A small compound library of 3,5-diaryl-2-methyl-1H-pyrroles, incorporating both electron- donating and electron-withdrawing scaffolds, was also synthesised under microwave irradiation using a solid- supported and multicomponent approach, albeit with low isolated yields. A few of these multisubstituted heterocycles were selected
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